The present invention relates to mixer circuits and, more particularly, to a mixer circuit with low noise and improved gain.
Mixer circuits, also known as modulator circuits, find many applications in electronic systems. For example, mixer circuits are widely used in wireless communication devices such as pagers and cellular phones to receive or transmit a modulated Radio Frequency (RF) signal. The function of a mixer circuit is to combine signals of two different frequencies in such a way as to produce energy at other frequencies. This function is typically achieved by designing a circuit that receives two input signals and produces an output that is product of the two inputs. The product of two periodic input signals will result in a modulated output signal as is well known in the art.
FIG. 1 illustrates a contemporary mixer circuit 100 for combining an RF signal and a differential input signal. The mixer circuit 100 of FIG. 1 includes a gain stage 110 and a bias circuit 120. The gain stage 110 of mixer circuit 100 receives a differential input signal (Vin1-Vin2) and produces a differential output signal V0. The gain stage 110 includes a pair of NMOS transistors 101 and 102 with their sources coupled together at common node 105. The gain stage 110 also includes a pair of load resistors 103 and 104 connected between the drains of transistors 101 and 102 and a supply voltage Vdd respectively. Bias circuit 120 of mixer circuit 100 includes an NMOS transistor 121 having an input to receive a bias voltage Vbias to produce a bias current Ibias. Bias current Ibias is coupled to the common node 105 to set the bias currents in transistors 101 and 102.
Mixer circuit 100 of FIG. 1 produces a modulated output by including an RF signal input to bias circuit 120. Bias circuit 120 receives an RF signal VRF at the drain of transistor 121. This signal has the effect of modulating the bias current Ibias. As Ibias is modulated, the bias currents in transistors 101 and 102 are modulated, thereby varying the gain of gain stage 110 by an amount proportional to the amplitude of the RF signal. Accordingly, the output of mixer circuit 100 is the product of the differential input voltage (Vin1-Vin2) and the RF signal VRF.
However, contemporary mixer circuits such as the one illustrated in FIG. 1 are often required to perform signal processing in environments that are very sensitive to the introduction of noise. For example, in a receiver the input signal may be very small (e.g. 10 microvolts). Accordingly, the receiver signal path requires high sensitivity, which in turn demands low noise. Therefore, a mixer circuit in the receiver signal path must minimize the amount of noise introduced into the system. Additionally, transmitted signals may be distorted by noise in the transmission signal path, and therefore can be more difficult to receive at the other end of the transmission medium. Likewise, signals input to a receiver may already be heavily distorted, and the introduction of additional noise may reduce the fidelity of the information contained in the signal. Moreover, the differential input signal (Vin1-Vin2) in many systems is generated on the same integrated circuit as the mixer circuit. Additional problems arise when the differential signal feeds into the RF signal path and is gained up with the RF signal in a preamplifier. Such feed-through distortion may lead to undesirable DC offsets.
Accordingly, a mixer circuit that reduces the amount of noise introduced into the signal path during either the reception or transmission of a signal is desired.